1. Field of the Invention
The present invention relates to recycling of vented atmosphere gas discharged from a heat treatment chamber utilizing a protective atmosphere gas therein.
2. Discussion of the Background
Metal parts are typically heat treated during the manufacturing process in order to achieve desired characteristics of the metal parts. Heat treating is usually conducted in a protective atmosphere in order to prevent adverse chemical reactions such as oxidation, carburization, hydrogen embrittlement, or nitriding. For most applications, an atmosphere that is chemically inert (e.g., argon, nitrogen for some metals) or chemically reducing (especially atmospheres that are partially or wholly hydrogen) is preferred. Hydrogen annealing, in particular, presents special advantages, as hydrogen has substantially better heat transfer properties and lower viscosity than other gases, and thus has allowed the development of the present state of the art high convection, bell annealing furnaces that mechanically circulate a hydrogen atmosphere around the metal being heat-treated to achieve even heating and freedom from chemical side reactions.
Since metals to be annealed have usually been mechanically-processed with oil lubricant, with or without subsequent washing, there is generally a large amount of contaminant in the form of oil and/or water present on the metal. Naturally, the annealing atmosphere is also initially inundated with atmospheric air, which contains oxygen, nitrogen and carbon dioxide in large amounts, all of which can cause undesirable chemical reactions as the metal is heated to the desired temperature for annealing, stress relief, etc. In present heat treatment facilities, the atmospheric air is displaced by a first purging gas, which may or may not be the final atmosphere gas, in order to displace the air and some vapour contaminants. Safety usually dictates that this first purging gas is not flammable. Alternatively, a mild vacuum may be applied in alone or in combination with inert gas to achieve the same ends.
As the metal is heated, the first purging gas may be continuously added to carry away evaporating oils and water, and prevent the admission of air, or a second atmosphere (usually hydrogen) may be introduced. Because the level of hydrocarbon, carbon oxide, nitrogen and water vapour impurities otherwise rises in the furnace atmosphere as the liquid materials vaporize and chemically decompose, the atmosphere is continuously removed from the heat treating furnace. Some commonly used strategies for disposing of the atmosphere gas are combustion at the vent (often referred to as “flame curtains”), collection and combustion as low-value fuel, or simply venting the atmosphere.
These methods of removing the impurities from the heat treating atmosphere suffer several drawbacks. Foremost is that these methods are wasteful of the atmosphere gas itself, which is usually costly. Even if recovered as a fuel, the fuel value of the recovered atmosphere gas is usually far lower than the cost of the pure atmosphere gas that is consumed. In the case of venting the atmosphere gas, the volatile organic compounds and carbon monoxide impurities in the gas are then dispersed in the ambient air, and form air pollution. At many locations, the emission of such pollutants is regulated, and extra emissions may be strictly prohibited, requiring the addition of expensive post-treatment equipment, such as thermal or catalytic oxidizers.
Methods to recycle metal treating atmospheres have been proposed. For example, U.S. Pat. No. 6,881,242 (hereinafter referred to as “the '242 patent”) to Krueger et al., proposes a system for hydrogen recycle where atmosphere gas is filtered, compressed, then supplied to a metal membrane hydrogen purifier. This method is limited in several respects. Because atmosphere gas is usually very hot, it is impractical to compress this gas to the pressures required in the '242 patent unless very small flows are contemplated. Further, the '242 patent provides no means for oil vapour removal before the separation step, meaning that the separator must operate while exposed to the mixed oil and contaminants from the decomposed metal forming oil, rinse solution, etc. Metal membrane purifiers are very sensitive to poisoning by sulphur, carbon deposition, etc., and thus are not usable for typical furnace atmospheres containing such compounds.
U.S. Pat. No. 5,348,592 (hereinafter referred to as “the '592 patent”) to Garg et al., presents another atmosphere recycling approach which is useful for removing water vapour and oxygen impurities only. Like the '242 patent, the '592 patent suggests that an adsorption gas dryer may be used to remove water vapor. In the case of the '592 patent, this water vapour is actually formed by catalytic reaction of trace oxygen with hydrogen as well as through pre-existing water vapour from the vented atmosphere gas. The catalysts used in the '592 patent are also very sensitive to deactivation by hydrocarbon vapors, by sulphur present in metalworking oils, and by carbon monoxide. Thus, the method of the '592 patent is not suitable for metallurgical atmospheres where these impurities are present. The '592 process is also incapable of ridding the vented atmosphere gas of unwanted inerts such as nitrogen, so is not capable of use when the atmosphere being recycled is desirably free of nitrogen, as in argon or hydrogen atmospheres.
Neither the '242 patent nor the '592 patent provide a means for excluding vented atmosphere gas during periods when impurity levels are such that they can not be effectively removed by the purification apparatus. Thus, during such periods, the vented atmosphere gas may cause equipment degradation or malfunction, or result in the recycle of substantial impurities to the heat treatment atmosphere.